The invention relates generally to holographic devices. The invention particularly relates to wavelength multiplexed holographic devices.
An alternative approach to traditional surface-based storage systems like compact discs (CDs) or digital versatile discs (DVDs) is volumetric storage technology, in which the full volume of a storage medium is used to increase data capacity. Holographic storage is one type of volumetric storage technology. Holographic storage has the potential to provide relatively high data density and short access times as compared to conventional optical storage technologies.
In conventional volume holographic storage, or page based holographic storage, laser light from two beams, a reference beam and a signal beam containing encoded data, overlap within the volume of a photosensitive holographic medium. The interference pattern resulting from the overlap of the two beams creates a change or modulation of the refractive index of the holographic recording medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages can be stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The reference beam interacts with the stored refractive index modulation and reconstructs the signal beam containing the encoded data. The parallel nature of this storage approach allows high transfer rates and short access times.
In bit-wise volume holography, data are stored bitwise in a photosensitive volume as microscopic reflection gratings called microholograms. A single micro-hologram corresponds to a single bit, where the presence or absence of a microhologram corresponds to a “1” or a “0” (or vice-versa). Overlapping microholograms can be stored in the same volume element by using multiplexing techniques, such as angle multiplexing or wavelength multiplexing. Such storage of multiple bits in the same volume element of the disk increases the storage capacity and potentially also the data transfer rates by the multiplex factor.
There is a constant requirement to find ways to increase the data storage density of holographic media. It is therefore desirable to find holographic systems and methods of using such systems, which help increase the data storage density.